Bolt doesn't move much during shot

[Shivesy]

First thing all my parts are new on this build except for my lower half. I've been out the last 2 evenings. The first evening (yesterday) I shot around 15 rounds and none of those rounds ejected and the bolt carrier never went back. Today I took apart the gas tube and gas block just to make sure that everything was lined up and I'm sure that everything there is fine. No obstruction anywhere and 3 to5 lbs of air flows through the system just fine. The bolt carrier lines right up with the gas tube and my carrier key is staked and is very solid on the bolt carrier. So this evening I made sure that each shot was shot with no rounds in the magazine which was installed in the mag. well. Not once has the bolt carrier moved more that 3/8" rearward.
My McGowen barrel has a .062 (1/6") gas port hole drilled into the barrel and I'm 99% sure that my gas block is in line with the gas port hole of the barrel. After each shot I look and check to see if the bolt has moved and yes it has but only about 3/8". I then take the charging handle, manually pull back on it and out comes the spent cartridge though the ejection port. The manuel ejection process seems to be of normal resistance. The gas rings on the bolt are aligned in a stair step manner. The only thing I can imagine is that maybe the barrel needs to have the gas port drilled larger but I'm not for sure. Any thoughts?

Thanks
Shivesy

[RangerRick]

Could there be an obstruction in the carrier key? If the gas isn't flowing through freely, it will cause problems.

[Shivesy]

I have 2 bolt carriers and blew through each one of the carrier keys. They both seem to be clear of any obstructions. I also took my air tool and shot 100 psi through the end of the key and again they do seem to be clear. Good point though, I didn't think of that one. I'm still leaning towards drilling out the barrel's gas port.

Thanks
Shivesy

[bwaites]

Check the gas block itself, check the gas port. Sometimes a little chip will get stuck in it.

Check the gas tube to make sure there is no pinch in it anywhere.

Call your barrel manufacturer and check the port diameter specs.

I would NOT drill the port until I was sure that it was the issue, and then I would use an adjustable gas block, just in case.

Do you have another lower? See if it functions on that lower if so.

[Shivesy]

First of all thanks for the replys. I talked to the gunsmith at McGowen barrels today. He stated that a .062 gas port hole is small but that's the way they ship there AR barrels out. If the bolt doesn't funtion then drill it out with a drill bit in 100 thousanths increments untill it functions.

I also talked to Robert Whitley from 6mmAR.com and he stated that 100 thousand increments seems to much becaue he works in 3 thousand increments. His guess would be a gas port size of .0850 to .0950 for a 20" barrel. He also questamated that a .0620 was to small of a gas port for this round. Robert states that when he custom builds his ARs that he starts with a smaller gas port size. Then he goes to the range with a cordless drill and some drill bits and the rifle. He takes a shot and if the bolt doesn't cycle then he opens up the gas port in 3 thousanths increments untill it cycles. Then once it cycles he goes 3 thousands more for good measure.

I think that this is the route that I'm gonna take. If anyone has any input please reply.

Thanks,
Shivesy

[Shivesy]

Check the gas block itself, check the gas port. Sometimes a little chip will get stuck in it.

Check the gas tube to make sure there is no pinch in it anywhere.

Call your barrel manufacturer and check the port diameter specs.

I would NOT drill the port until I was sure that it was the issue, and then I would use an adjustable gas block, just in case.

Do you have another lower? See if it functions on that lower if so.

Thanks for the reply,

I have my adjustable gas block opened up all the way and this is the only lower (Rock River) that I have and it works just fine with my RR 223 20" upper. Everything is new and clear. Gas block, gas tube, carrier key, buffer and spring ect. The spent cartridge is removed by the charging handle with no excessive force.

[glane5910]

I'd have to agree with Robert. That's seems way too small of a gas port. I had the basic same problem with a 338 Federal DPMS upper. They drilled the port the same size that a 308 should've been. The 338 Federal needs larger. I drilled it out and it runs smooth as silk. I'd go with the 0.085" unless you can prove the gas block is leaking.

[Shivesy]

This is a new .75 adj. gas block that fits pretty darn tight. I have to carefully twist it back and forth to install it. I don't think that it's leaking.

[glane5910]

I'd have to go with the too small of a gas port then. 2 things might've happened.
1. They drilled the wrong size on all of their barrels.
2. They drill with the smaller bit then work up to the final size and this one just didn't get drilled larger.

[warped]

Can you plug the chamber with a spent cartridge and blow through the muzzle and have it come out the gas tube?

[Shivesy]

Can you plug the chamber with a spent cartridge and blow through the muzzle and have it come out the gas tube?

I just put a spent cartridge in the chamber and closed the bolt. I cannot blow through the barrel. If I put a airhose (20 psi) with a rubber tip nozzle to the end of the barrel YES air comes out of the gas tube inside the upper by the barrel nut.

[Shivesy]

Well I finally had a chance to go out to the range tonight and took a cordless drill and some drill bits with me. I took off the gas block along with the attached gas tube and drilled the gas port hole from .062 (1/16") to the next size bigger in my drill box .077 (5/64"). I put the rifle back together, cleaned the barrel real good, I put one round in the chamber and loaded an empty magazine into the mag well. I took a shot and the bolt ejected the round but would not lock back with the empty mag. I knew I was on the right track so I removed the gas block and tube again and drilled the gas port out to .093 (3/32"), did a good cleaning again and fired one shot. This time the bolt ejected the round and locked backed with the empty magazine. I shot off about 10 rounds before dark and this rifle worked flawlessly. I wasn't able to group real good because I was just shooting off my shoulder. All the brass looked good on the out side but I did notice that a few of the empty cases were slightly crimped at the neck of the spent cartridge. I'm glad that this rifle will now cycle and will post some groups later. This is a 20" barrel and .093 is my gas port hole size.

[glane5910]

good deal.. That port size was pretty small.

[DirectDrive]

I talked to the gunsmith at McGowen barrels today. He stated that a .062 gas port hole is small but that's the way they ship there AR barrels out.

He's thinking 5.56.
You will probably end up around .090 once you get it worked out.

[RangerRick]

All the brass looked good on the out side but I did notice that a few of the empty cases were slightly crimped at the neck of the spent cartridge.

If the case mouth was flattened on one side it was from hitting the brass deflector. You can put a piece of stick-on velcro on it (the fuzzy side) to pad it. It will reload more times that way.

If you don't reload sell the brass to someone on here.

Glad you got it working!

Rick